It's sort of ironic that you talk about hiring a professional then mention 99designs.
I understand the appeal and why it works for some applications, but you're missing out on the best parts of working with a designer by doing that (not to mention you're probably working with a hobbyist and run the risk of getting a recycled/stolen logo).
I'm a software developer, so I know my design limitations.
I also know the mistakes and problems caused by clueless buyers of design services a.k.a. Pointy-Haired Bosses (ask for bitmaps, and way too many designers will eagerly comply...) I am tired of explaining One More Time that I cannot take your 100x100 iPhone app icon done as a JPG and make it look good. I get tired of asking "where are the source images" and the inevitable "what is that?"
I've worked with some of the best in-house designers when working for others (MediaShare, which was bought by MacroMedia which was bought by Adobe..., Sony (Playstation) etc. So, I know what great professional designers given an essentially unlimited budget can do, thank you!
(And even working with the best - some are willing to listen, and willing to constrain their work within the bounds of what is technically feasible - and some are not. I suppose this is less of an issue today - back in the early 90's too many wanted to do beautiful airbrush-like Photoshop renderings that.... would take a minute to load. And you just couldn't convince them that this is Not a Good Thing...)
And I've worked with local freelancers, and, frankly it was a pain.
I've done two 99Design projects - my company logo and business card, and some modifications to an app background that was done by a local freelancer.
In both cases, 99Design was a breath of fresh air.
First off, there is the aspect of having a wide choice of design ideas, rather than have one designer's take, which is typically whatever is currently scratching their itch. Sure, I got a lot of junk, which I quickly rejected.
It is really important to engage and give feedback when working with 99Design etc. Probably most buyers don't put a sufficient level of effort into it, though, from looking over projects there.
Actually, I've done 3 projects, as I decided the logo I went with was not what I really wanted, and I really liked a more whimsical one that had been runner-up. I went back to that designer 2 years later (through 99 design) and had them complete the project. Fortunately, they had saved their sketches!
Of course, you have to vet them. Which you need to do anyway, regardless of the source.
What are the "best parts of working with a designer" that I am missing. Given that I've been there, done that? Maybe I am missing something.
I would much rather 99Design than DIY, and suspect it is a better choice for most, unless you have some artistic talent and interest. But if you did, you wouldn't be here asking what tools to use - you would know.
I might be in a fairly unique position of knowing what to look for when I work with a site like 99Design. I can see how if you didn't know the right questions to ask and the right directions to give, you could wind up with a useless mess.